I suspect there's no absolute correct answer to your question which boils down to; is it best to run low on oil than to top up with incorrect specification oil? If I had to guess I'd say the wrong oil is better than no oil at all.
Alternatively, it's likely you'll be topping up your oil in the the future again. So for ~£10 go to Halfords (or similar) and buy 1L of correct oil for now and the future.